> Maybe it's the display, since I still only have the old 1600x panel.
It is most likely due to poor quality batteries. The "cells" in a 9-cell battery are 18650 batteries, which vary in capacity from 3400mAh at the high end, to counterfeit ones that are labelled to have 1200mAh capacity, but in reality have only a fraction of that. Lithium-ion batteries also have a limited shelf life, so even good quality aftermarket batteries will have a fraction of the rated capacity if they have been sitting in a warehouse for years.
It is most likely due to poor quality batteries. The "cells" in a 9-cell battery are 18650 batteries, which vary in capacity from 3400mAh at the high end, to counterfeit ones that are labelled to have 1200mAh capacity, but in reality have only a fraction of that. Lithium-ion batteries also have a limited shelf life, so even good quality aftermarket batteries will have a fraction of the rated capacity if they have been sitting in a warehouse for years.